Known items of equipment of this type are in particular described by patent application or patent publications no. WO 98/55038 and U.S. Pat. No. 8,308,775, or by French patent application publication no. FR 2,856,581 A1. In these items of equipment, each bone anchoring member is in the form of a “polyaxial” pedicle screw, i.e., comprising an articulated proximal threaded pin; the corresponding connecting part is designed to be engaged on that proximal threaded pin using a hole that it comprises; said tightening means is made up of a nut designed to be screwed on the threaded proximal pin and to bear against the connecting part. Tightening the nut makes it possible to pull the screw toward the connecting part until a bearing surface arranged on the screw comes into contact with the connecting part. This gradual pulling makes it possible to correct the position of the vertebrae. Once said bearing surface is placed against the connecting part, the nut is pulled tight and the part of the pin protruding past the nut is broken. To favor this breaking, the pin comprises a thinner portion with a lower resistance; after breaking, the threaded proximal pin is reduced to a remaining portion of that pin.
With such items of equipment including pedicle screws, positioning the screws perfectly in the pedicles of the vertebrae is problematic. Indeed, if a screw deviates from its correct position, it may cause compression or even damage of a nerve ending, creating significant pain for the patient. In such a case, it is necessary to operate on the patient again in order to correct the position of the screw. This correction may, however, lead to a noticeable change in the position of the screw with respect to the corresponding connecting part, making it difficult or even impossible to resituate the nut on the remaining portion of the threaded pin.
The same problem of resituating the nut exists when the screw comprises a short, unbreakable proximal pin, i.e., an item of equipment whose reduction capacity is essentially based on the ancillary associated with that item of equipment.
The aforementioned documents, and in particular document FR 2,856,581 A1, do not address this specific problem and therefore do not provide a solution thereto. In the absence of such a solution, either one may decide not to correct the incorrect position of the bone anchoring member, or it is necessary to replace the entire item of equipment, which involves a long and complex procedure.